1. Technical Field
The present invention relates to an apparatus and a method for electrically detecting the displacement of a magnet moved in response to variation of a physical quantity, the method being applicable to measuring instruments such as flowmeters or level gauges, more particularly to a displacement detecting method and apparatus using the same for transforming the displacement of the magnet through hall devices into a corresponding electrical signal.
2. Description of the Prior Art
In the art of industrial flowmeters or level gauges, used broadly are displacement detecting apparatus of a type including a magnet adapted to be displaced within a sealed chamber in response to the variation of the measured physical quantity (e.g. the flow rate or the height of the liquid). The measurement of such displacement of the magnet is effected externally of the chamber. Such a detecting apparatus detects the magnetic lines of flux from the moving magnet.
One example of an area flowmeter including the above mentioned detecting apparatus of the prior art is illustrated in FIG. 17. The flowmeter includes a measuring tube 31, a moving element 32 (referred generally to be a float) adapted to be moved vertically within the tube in response to the variation of the fluid to be measured, a magnet 33 mounted within the moving element, and hall device 34 provided around the measuring tube 31.
The density of the magnetic flux 35 from the magnet 33 and permeating through the hall device 34 varies upon moving the magnet 33 vertically in accordance with the variation of the flow rate of the fluid to be measured. Thus the flow rate of the fluid or the displacement of the moving element can be determined by the output voltage from the hall device varying in accordance with the variation of the density of the magnetic flux.
However, in the detecting apparatus of the type as mentioned above, the larger the distance between the hall device 34 and the magnet 33, the lower the output voltage from the hall device. This will increase the measuring error, so that the correct detection on the position can not be expected. In other words, the detecting apparatus of the prior art can be used only in the case that the range of the movement of the magnet is limited.
The influence of the variation of temperature and that with time on the output of the hall device 34 and/or on the density of the magnetic flux generated from the magnet 33 will also be the obstruction against the proper detection.
The detecting apparatus of the other prior type including a plurality of hall devices provided linearly and in parallel with the direction of the movement of the magnet can also be used.
However, in such apparatus, each hall devices are used as the proximity switch, and the position of the magnet is adapted to be detected on the basis of the position of the hall device outputting the voltage higher than the predetermined threshold value.
The detected value on the position of the magnet will be output in the form of the digital data, so that the influence caused by the variation of temperature and that with time will be improved. Thus, the disadvantages inherent in the apparatus shown in FIG. 17 are improved. Nevertheless, the resolution of the apparatus of the prior art is merely about equal to or half of the spacing between the magnetic sensors such as hall sensors. In this connection, if it is desired to detect the displacement more precisely, it is necessary to increase the number of the hall device and to make the distance between adjacent hall devices narrower. This will also increase the cost for the apparatus.
The spacing between adjacent hall devices must be sufficiently larger relative to the size of each hall device, so that it is necessary to make the amplitude of the displacement of the magnet larger in order to provide a plurality of hall devices to achieve high resolution.
Accordingly the object of the present invention is to provide an apparatus and a method for detecting the displacement of a magnet solved in the problem inherent in the apparatus of the prior art. In accordance with the present invention, the errors on the detected value due to the variation of temperature or that with time are hardly caused in the apparatus. The apparatus can detect the displacement in high resolution and precision. The apparatus is simple in its structure, and it can be expected that the apparatus can be manufactured in low cost. The apparatus is not limited in its application by the range of the displacement of the magnet.
These and other objects are achieved by a method, and an apparatus employing the method, for detecting the displacement of the magnet, the method being characterized by disposing a plurality of hall devices in a predetermined spacing along a path of displacement of the magnet in parallel thereto, each of the hail devices including a magnetically sensitive surface through which a magnetic flux from said magnet permeates to generate an output signal having a polarity and magnitude which are dependent on the direction and the density of the magnetic flux, and wherein said magnetically sensitive surface of each said hail device is disposed in parallel with the direction defined by the magnetic poles of said magnet, detecting two adjacent hail devices having output signals of opposite polarities to determine the general position of the magnet, and then determining the position of a center line of the magnet between these two hail devices on the basis of the magnitudes of the output signals of these two hall devices.
The method is further characterized by disposing a plurality of hail devices in a predetermined spacing DP along a path of displacement of the magnet in parallel thereto, each of the hall devices including a magnetically sensitive surface through which a magnetic flux from said magnet permeate to generate an output signal having a polarity and magnitude which are dependent on the direction and the density of the magnetic flux, and wherein said magnetically sensitive surface of each said hall device is disposed in parallel with the direction defined by the magnetic poles of said magnet, detecting a pair of adjacent (n)th and (n+1)th hall devices (n=0, 1, 2, . . . ) having output signals V(n) and V(n+1) of opposite polarities to determine a general position of the magnet, and then determining a position of a center line of the magnet MP relative to a position of (0)th hail device as a reference point on the basis of the following equation
MP=nxc2x7DP+[V(n)/{V(n)xe2x88x92V(n+1)}]xc2x7DP. 
An apparatus in accordance with the method of the invention is characterized by an arrangement which comprises a plurality of hail devices disposed in a predetermined spacing DP along a displacing path of the magnet in parallel thereto, wherein two adjacent (n)th and (n+1)th hail devices (n=0, 1, 2, . . . ) inverted in the polarity of their output voltages V (n) and V (n+1) are detected to determine the general position of the magnet, and the precise position of the magnet between these two hail devices is determined on the basis of the following equation;
MP=nxc2x7DP+[V(n)/{V(n)xe2x88x92V(n+1)}]xc2x7DP 